The present invention generally relates to an apparatus and a method for reducing particle contamination in an etcher that utilizes a ring member positioned juxtaposed to a wafer holder and more particularly, relates to an apparatus and a method for reducing particle contamination from a polymeric film adhered to a focus ring used in a reactive ion etching apparatus.
In the fabrication of semiconductor devices, particularly in the fabrication of submicron scale semiconductor devices, profiles obtained in etching processes are very important. A careful control of the surface etch processes is therefore necessary to ensure directional etching. In conducting an etching process, when an etch rate is considerable larger in one direction than in the other directions, the process is called anisotropic. A reactive ion etching (RIE) process assisted by plasma is frequently used in anisotropic etching of various material layers on top of a semiconductor substrate.
The plasma generated in a RIE process consists of neutrons, ions and electrons. The effect of each species on the etch results and their possible interactions with each other are not well understood. An etch process and the resulting etch products can be analyzed by mass spectrometry, while the chemical compositions on the surfaces can be detected by an in-situ or ex-situ x-ray photoelectron spectrometer, Auger spectroscopy and secondary ion mass spectrometry. In plasma enhanced etching processes, the etch rate of a semiconductor material is frequently higher than the sum of the individual etch rates for ion sputtering and neutral etching due to a synergy in which chemical etching is enhanced by ion bombardment.
In an anisotropic etching process, it is clear that the ions striking the surface must themselves be anisotropic, i.e., the ions travel mainly in a direction perpendicular to the wafer surface. The ions are normally oriented and accelerated in a sheath. Proposed mechanisms for anisotropic plasma etching are that first, perpendicular ion bombardment creates a damaged surface that is more reactive toward neutral etchants, and second, ions help to desorb etch-inhibiting species such as etch products from the surface. In either one of the mechanisms, the ion path must be perpendicular to the wafer surface such that only the etch rate of the bottom surface is enhanced. Ideally, the ions should not bombard the sidewalls at all.
In an actual plasma etching process, ion-neutral particle collisions from the plasma sheath result in a fraction of the ions bombarding the sidewalls. As a result, lateral etching of the sidewalls occurs to some extent. Theoretically, the number of ion-neutral collisions in the sheath is directly proportional to the sheath thickness and inversely proportional to the ion mean free path. Since the ion mean free path is usually proportional to the chamber pressure, reducing the pressure results in reduced ion-neutral collisions and therefore enhances anisotropic etching. Since ions desorb etch-inhibiting species (such as etch products) from the etch surface, the formation of sidewall films (i.e., a passivation layer) during the etching process plays an important role in the development of the anisotropic etch profile. The passivation film protects the sidewalls from etching by non-perpendicular incoming ions.
In an anisotropic etching process, the etch directional control can be enhanced by a mechanism known as sidewall passivation. By adjusting the etchant gas composition and the reactor parameters, an etch-inhibiting film can be formed on the vertical sidewalls. The etch-inhibiting film (or passivation film) slows down or completely stops lateral attack while the etching of horizontal surfaces (i.e., a bottom surface) proceeds. For instance, in an etch process for silicon, when O2 is added to a Cl2 plasma, an oxide film can be grown on the sidewalls that are not exposed to ion bombardment. Similarly, in a fluorocarbon plasma etching process, a greater elemental ratio of carbon to fluorine can be used to deposit involatile polymer films on the sidewalls thus forming a coating that blocks chemical attack. While polymer film may also deposit on the horizontal surfaces, it is readily removed by the ion bombardment and therefore allowing etching of such surfaces to continue.
The sidewall passivation effect is also observed when an insulating layer such as a photoresist layer deposited on top of a semiconductor substrate is bombarded by plasma ions. On top of the photoresist layer, charge built up occurs during the reactive ion etching process by the severe ion bombardment on the substrate surface. These stored charges can cause a distortion in the ion path bombarded toward the substrate surface. When positive charge accumulates on the wafer surface as a result of impinging ions and emitted secondary electrons, the photoresist surface may be charged up high enough to produce a current flowing through the photoresist layer causing its degradation or other permanent damages. The charge accumulation also causes a distortion in the path of the ion beam by stripping the space-charge compensating electrons from the ion beam. Such charge accumulation further distorts the ion beam path and causes them to collide with the sidewalls of the device.
In a plasma bombardment process conducted on a semiconductor structure covered by a photoresist layer, the plasma ions excited by the bias voltage etch away difficult-to-etch residue materials from the semiconductor substrate, while simultaneously etches a fraction of the photoresist material from the photoresist layer to perform sidewall passivation in order to enhance the anisotropic etching process. In the process where the photoresist material is etched away from the top of the substrate, two steps are normally involved. In the first step, the photoresist material is etched away and separated from the photoresist layer and deposited on the top portion of the sidewalls. The deposited photoresist material is then bombarded again by the plasma ions and sputters to the lower portion of the sidewalls. Since photoresist layer normally consists of a polymeric material and that when it decomposes in plasma, fragments of the polymeric material combine with some of the gas elements in the plasma to form a passivation material for depositing on the sidewalls.
When a plasma etching process on the surface of a silicon wafer is conducted, for instance, in a process for forming a STI (shallow trench isolation) in a polysilicon layer that is defined by a photoresist on top, the polymeric material generated during the etching process not only deposits on the sidewalls of the shallow trench as it is supposed to, but also deposits on other areas inside the etch chamber other than the wafer surface. For instance, the polymeric material may form a thin film on the chamber wall and on a focus ring that is normally positioned juxtaposed to the wafer pedestal for confining a plasma cloud to the wafer surface. After repeated etching process on numerous wafers is conducted in the same etch chamber, the polymeric film deposited on the various components in the etch chamber may be cumulated to a significant thickness and as a result, the probability of the film peeling off from the surface it attached to becomes significantly higher. When such peeling of the polymeric film occurs, a serious contamination problem resulted when the film falls on the wafer surface.
In the plasma etch chamber, while certain components including the chamber wall stays stationary during the etching and the wafer loading/unloading process, certain other components move up and down with the wafer pedestal during the wafer loading/unloading process. One of such components that moves with the wafer pedestal is a focus ring which is made of a quartz material to endure the highly corrosive environment of the etching plasma. The focus ring is a circular ring member that is positioned just above the wafer pedestal and the wafer, serving the function of confining a plasma cloud to the exposed surface of the wafer. Since the focus ring is placed in close proximity to the wafer surface and to the plasma could, it is also the most likely place where polymeric film deposit occurs. During the up and down movement of the focus ring with the wafer pedestal, the deposited polymeric film flakes off the surface of the focus ring and becomes a serious contamination source. Since the formation of the polymeric material is desirable for the plasma etching process, i.e., in order to achieve the desirable anisotropic etching, the deposition of the polymeric material on the focus ring cannot be avoided. It is therefore necessary to provide a method for preventing the polymeric film layer deposited on the focus ring from flaking-off and becoming a contamination source to a wafer.
It is therefore an object of the present invention to provide an apparatus and a method for reducing particle contamination in a plasma etcher that do not have the drawbacks or shortcomings of the conventional apparatus and method.
It is another object of the present invention to provide an apparatus for reducing particle contamination in a plasma etcher wherein the formation of a polymeric film during the etching process as a contamination source can not be avoided.
It is a further object of the present invention to provide an apparatus for reducing particle contamination in a plasma etcher wherein the formation of a polymeric film for sidewall passivation is necessary and the deposition of the polymeric material on chamber components is inevitable.
It is another further object of the present invention to provide an apparatus for reducing particle contamination in a plasma etcher wherein the deposition of a polymeric film on a moving chamber component of a focus ring is inevitable.
It is still another object of the present invention to provide an apparatus for reducing particle contamination in a plasma etcher by providing a focus ring which is fabricated of a quartz material with a roughened surface for improving adhesion of a polymeric film thereto.
It is yet another object of the present invention to provide an apparatus for reducing particle contamination in a plasma etcher wherein a focus ring is provided with a roughened surface such that the adhesion of a deposited polymeric film thereon is improved for preventing flaking of the film as a contamination source.
It is still another further object of the present invention to provide a method for dry etching a wafer without contamination problem by using a focus ring with a roughened surface which has a depth between peaks and valleys in the surface between about 1 xcexcm and about 10 xcexcm.
It is yet another further object of the present invention to provide a dry etching apparatus for etching a wafer wherein a focus ring is provided with a roughened surface that has depth between about 1 xcexcm and about 10 xcexcm between peaks and valleys formed in the roughened surface for achieving improved adhesion with a polymeric film and for preventing the flaking-off of the film.
In accordance with the present invention, an apparatus and a method for reducing particle contamination in a plasma etcher is provided.
In a preferred embodiment, a dry etching apparatus for a wafer that has reduced particle contamination characteristics is provided which includes a wafer holder for holding a wafer inside a chamber cavity and a ring member positioned juxtaposed to the wafer holder, the ring member has surfaces substantially exposed to the chamber interior, the surfaces is roughened to a depth of between about 1 xcexcm and about 10 xcexcm between peaks and valleys formed in the roughened surface.
In the dry etching apparatus for a wafer that has reduced particle contamination characteristics, the ring member may be fabricated of a high temperature insulative material, or of a ceramic material, or of a quartz. The ring member may be a focus ring for a reactive ion etcher. The roughened surface may have a depth preferably between about 3 xcexcm and about 5 xcexcm between peaks and valleys formed in the surface. The roughened surface may be formed by sand-blasting with particles of 80 mesh or smaller, or with particles between 200 mesh and 80 mesh. The ring member confines a plasma cloud generated in the chamber cavity onto an exposed surface of the wafer.
The present invention is further directed to a method for dry etching a wafer without polymer contamination problem which can be carried out by the operating steps of providing an etch chamber that is equipped with a wafer holder therein, providing a ring member for positioning juxtaposed to the wafer holder when a wafer is positioned thereon, roughening surfaces on the ring member until a depth between peaks and valleys in the surfaces between about 1 xcexcm and about 10 xcexcm is obtained, and igniting a plasma in the etch chamber to etch the wafer.
The method for dry etching a wafer without polymer contamination problem may further include the step of roughening surfaces on the ring member until a depth between peaks and valleys in the surfaces preferably between about 3 xcexcm and about 5 xcexcm is obtained. The method may further include the step of roughening surfaces on the ring member by sand-blasting with particles of 80 mesh or smaller, or by sand-blasting with particles between 200 mesh and 80 mesh. The method may further include the step of providing the ring member in a quartz material, or providing the ring member for use as a focus ring in a reactive ion etcher. The method may further include the step of positioning the ring member over and juxtaposed to the wafer such that plasma generated in the etch chamber is confined to an exposed surface of the wafer. The method may further include the step of adhering a polymer material generated by the plasma to the roughened surfaces of the ring member such that it does not fall off the roughened surfaces to become contaminant.
In another preferred embodiment, a dry etching apparatus for etching a wafer is provided which includes a wafer holder for holding a wafer inside a chamber cavity, a ring member for positioning juxtaposed to the wafer holder for confining a plasma generated in the chamber cavity onto an exposed surface of the wafer, the ring member has surfaces substantially exposed to the chamber interior, the surfaces being roughened to a depth of between about 1 xcexcm and about 10 xcexcm between peaks and valleys formed in the roughened surfaces for achieving improved adhesion with a polymeric film deposited thereon and for reducing flaking-off of the film as a contaminant onto a wafer positioned on the wafer holder, and a plasma source for generating a plasma in the chamber cavity.
In the dry etching apparatus for etching a wafer, the surfaces are preferably roughened to a depth of between about 3 xcexcm and about 5 xcexcm between peaks and valleys formed in the roughened surfaces. The ring member may be a focused ring made of quartz for use in a reactive ion etcher.